Conservation of Mass & Rate of a Chemical Reaction

Conservation of Mass & Rate of a Chemical Reaction Sec. 4.2 & 4.3p. 163-170

Sec. 4.2 - Conservation of Mass in Chemical Reactions • When a chemical reaction occurs, products are formed from the reactants. • These products often look very different than the reactants.

MASS OF REACTANTS = MASS OF PRODUCTS • HOWEVER, the total mass of these products always equals (is the same as) the mass of the reactants. • This is called the conservation of mass. • No matter is created or destroyed in chemical reactions. • Ex: Rocket science – -was there matter created or destroyed? Why did the substance no longer fit in the container?

Closed Systems • An experiment in which no additional material is allowed in or out. • When in a closed system, the mass of all reactants always equals the mass of all the products. • Example: 13.9g of Fe(s) + 8.0g of S(s) 21.9g of FeS(s). 8.2g of Na + _____ of Cl  13.0g of NaCl 4.8g

More examples… • 1) In a closed system, the total mass of the reactants is 15.8g. What is the mass of the products? _______________ • 2) 25g of one substance reacts with 14g of another substance in a closed system. What will the total mass of the products be? ___________ 15.8g 25g + 14g = 39g

Open systems • Some reactions do not seem to follow this principle. • An Open System is • If one or more of the products have escaped we cannot measure the final mass of the products accurately. An experiment in which one or more products of a chemical reaction can escape.

Example… • NaCHO3 (s) + C2H4O2 (l) NaC2H3O2 (aq) + H2O(l) + CO2 (g) • Rocket science • If we measured the mass of what was left on the launch pad, it would not equal the mass of the reactants. • What were the reactants? • What were the products? • Why can we not measure the products mass accurately?

Assignment: • Page 165 • Questions 5, 6, 7, 9

Sec. 4.3 – Factors affecting the Rate of a Chemical reaction It is important to understand how a chemical reaction works and the factors that affect it. Why? (baking a cake, coloring your hair…)

The four main factors that affect a chemical reaction are: • Presence of a catalyst • The concentration of the reactants • The temperature of the reactants • The surface area of the reactants

1. Catalysts • Are substances that speed up a reaction. • Are present during the reaction, but not consumed. • Example: • ENZYMES! They help break down food. Without an enzyme we would need really high temperatures to break down food, high temp would be deadly. • http://www.youtube.com/watch?v=ezsur0L0L1c Elephant toothpaste • http://www.youtube.com/watch?v=608OIFWYnto Steve Spangler

2. Concentration • The greater the concentration of reactants, the FASTER the reaction. • Example: • Rocket science – by adding more alka seltzer, the reaction occurred faster and in response went higher. • Varying solution concentrations in a reaction. The strongest solution will proceed the fastest.

3. Temperature • The HOTTER the reactants, or more heat added, the FASTER the reaction. • Example: if we heated the vinegar first, the reaction would proceed faster.

4. Surface area • INCREASED surface area causes a reaction to occur FASTER. • More surface of reactant means more area for the reaction to occur. • Example: Rocket Science – if we crushed up the alka seltzer into a powder, what would happen? The reaction would speed up

Assignment: • AYL p. 171 • Questions: 1-3, 5-9

Conservation of Mass & Rate of a Chemical Reaction